As a flaw detection method for obtaining information on flaws or defects of a target has been conventionally known an immersion type automatic flaw detection method as disclosed in a journal of the Japan Society of Mechanical Engineers, Vol. 90, No. 826, pp 5-9 (hereinafter, referred to as "prior art 1"), or in a journal of the Japanese Society for Non-destructive Inspection, Vol. 37, No. 2, pp 152-153 (hereinafter referred to as "prior art 2"). These types of flaw detection methods are used to perform precise flaw detection using ultrasonic wave, and mainly includes a process in which a target immersed in water is subjected to flaw detection while an ultrasonic probe is scanned over the surface thereof.
In order to obtain an accurate information on the size and position of a flaw (defect) of the target using the immersion type of automatic flaw detection method as described above, the following two conditions are required. One is that a distance between the ultrasonic probe and the target is kept constant, and the other is that the direction of a center axis of an ultrasonic beam is coincide with the direction of a normal to the surface of the target.
In the prior art 1, the flaw detection can be performed for a target having a plane surface, but not for a target having a curved surface because it is difficult to coincide the direction of the central axis of the ultrasonic beam with the normal direction to the surface of the target. That is, the prior art 1 never satisfies the latter condition for the target having an curved surface.
On the other hand, the prior art 2 includes two steps for the flaw detection, in which the whole shape or profile of a target is beforehand measured using a laser range finder to obtain an information on the shape or profile of the target, and then an ultrasonic probe is scanned in accordance with the information. As described above, this prior art 2 includes the step of beforehand measuring the shape or profile of the target, and thus can perform the flaw detection for an target having any shape or pro-file. This type of ultrasonic flaw detector has been disclosed in Japanese Unexamined Published Patent Applications No. 63-309852 and No. 63-309853 (hereinafter referred to as "JPAs").
However, the prior art 2 and the ultrasonic flaw detector as disclosed in the "JPAs" have the following problems.
As described above, these flaw detectors require a multi-scanning (two-step scanning) operation of the surface of the target using a scanning mechanism, that is, both of a first scanning operation using a laser beam and a second scanning operation using an ultrasonic beam, and thus the flaw detection needs 2 times as long as a time ordinarily required for the flaw detection. In addition, the shape measurement of the target by the laser range finder must be made in air while the ultrasonic flaw detection must be carried out in water, so that the environmental equipment for the whole flaw detection process is complicated and gigantic in construction, and a long time is required for the flaw detection.
It is an object of this invention to provide an ultrasonic flaw detector which can shorten a time required for the whole flaw detection process and improve the accuracy of the flaw detection.